(a) Technical Field
The present invention relates to a driving force control of a hybrid electric vehicle. More particularly, the present invention relates to a system and method for controlling a driving force of a hybrid electric vehicle which can reduce impact shock during clutch engagement by starting an engine while the hybrid electric vehicle is driven by a motor-generator.
(b) Background Art
In general, a driving system of a hybrid electric vehicle includes an engine, a motor-generator, and a clutch for controlling connection and disconnection between the engine and the motor-generator. While the vehicle is driven by the engine or by both the engine and the motor-generator, the clutch is in a connected state or in a slip state. Meanwhile, while the vehicle is driven only by the motor-generator without using the engine, the clutch is opened to disconnect the engine from the motor-generator. If there is the necessity of using the driving force of the engine while the vehicle is driven only by the motor-generator, that is, if it is more advantageous to drive the vehicle in combination with the engine and the motor-generator at a high speed or in a case where a driver abruptly depresses an accelerator pedal, the engine is started and the clutch is connected thereto, thus using the driving force of the engine.
During the clutch engagement, if the rotational speed of the engine and that of a clutch plate are different from each other, impact or noise is generated in the vehicle. The engine has a low response to a control command. As a result, the rotational speed of the engine may be rapidly increased with a time lag, if the output is increased in a state that the engine operates at a low load, making it difficult to adapt the rotational speed of engine to a high rotational speed upon start-up thereof. Thus, there has been a need for a device or method for preventing an impact caused by the clutch engagement during transition from an electric motor running mode to an internal combustion engine running mode in the conventional hybrid electric vehicle.
Japanese Patent Publication No. 2006-347431 discloses a method for controlling rotational speed and torque of an engine by means of fuel cut, ignition timing delay, or throttle opening adjustment during clutch engagement. The method, however, has a drawback. For example, control of the rotational speed is insufficient. Furthermore, a difference between the rotational speed of engine and that of clutch may be increased, thus causing vibration and noise during the clutch engagement.
Japanese Patent Publication No. 2006-054940 discloses a method for controlling a rotational speed of an engine during start-up thereof using a motor. The method, however, has a drawback. For example, it includes a centrifugal clutch automatically engaged when the rotational speed of the engine is increased, the motor only functions to supplement an insufficient engine torque upon start-up, making it impossible to accurately control the rotational sped and torque of the engine and thus causing vibration and noise during the clutch engagement.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.